1. Field of the Invention
The invention relates to an image display unit for displaying an image on a display device in a plurality of sub-fields, wherein the display device is capable of generating, in each of the sub-fields, a respective illumination level, the image display unit comprising selection means for selecting a first combination of sub-fields for displaying a first color sub-pixel of a particular pixel with a first intensity level, and for selecting a second combination of sub-fields for displaying a second color sub-pixel of the particular pixel with a second intensity level.
The invention further relates to an image display apparatus comprising such an image display unit.
The invention also relates to a method of displaying an image on a display device in a plurality of sub-fields, whereby the display device is capable of generating, in each of the sub-fields, a respective illumination level, the method comprising a step of selecting a first combination of sub-fields for displaying a first color sub-pixel of a particular pixel with a first intensity level, and of selecting a second combination of sub-fields for displaying a second color sub-pixel of the particular pixel with a second intensity level.
2. Description of the Related Art
European Patent Application EP 0 896 317 A2, corresponding to U.S. Pat. Nos. 6,014,258, 6,208,467 and 6,518,977, describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a large number of cells that can be switched on and switched off. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase, in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for zero, one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it is switched on, is integrated in the eye of the viewer. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Different sub-fields may have a mutually different or equal duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32, respectively. This is a so-called binary distribution. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using code words of 6 bits each, whereby a code word indicates, in binary form, which sub-fields are to be switched on, i.e., what the intensity level of a pixel is.
The device described in EP 0 896 317 A2 uses a non-binary distribution of the sub-fields weights. Compared with the binary distribution, the relatively high valued sub-fields of the binary distribution have been split into two lower valued sub-fields. This is at the cost of a reduced number of intensity levels that can be realized with a given number of sub-fields, or at the cost of an increased number of sub-fields for realizing a given number of intensity levels. In the known device, almost every intensity level can be realized by a combination of a high and a low sub-field. In this way, a continuous gradation can be represented with a reduction of false contour interference. In a particular embodiment, the device has two tables, each one of which indicates, for each possible intensity level, the combination of sub-fields realizing that intensity level. For a number of intensity levels, the combination indicated in the one table for a specific intensity level is different from the combination indicated in the other table for that specific intensity level. It is proposed to apply a checkerboard pattern to the image and to use, for a pixel from a white block of the pattern, the combinations from the first table, and for a pixel from a black block of the pattern, the combinations from the second table. This results in a further reduction of false contours.
It is at object of the invention to provide an image display unit as described in the preamble, with a reduction of flicker. This object is achieved, according to the invention, in a display unit that is characterized in that the selection means is arranged to select that combination as second combination in which the subjective peak in luminance is at a different time position in the frame period compared with the subjective peak in luminance in the first combination. By controlling that the luminance peak from one color sub-pixel falls at a different moment than the luminance peak from the other color sub-pixel, the frequency component of the pixel signal having the frame frequency, usually 50 Hz or 60 Hz, is reduced. By a proper selection of the combination of sub-fields, the peak generated for one color sub-pixel is compensated by a peak in the other color sub-pixel. This is because the two sub-pixels are close together and that, as a consequence thereof, they are perceived as a single light source. It has appeared that when one color is generated at one instant in the frame period and the other color at a later instant in the frame period, the color perception remains unaffected compared with the simultaneous generation of the colors. Hence, one sub-pixel is lit early in the frame period and the other sub-pixel is lit later in the frame period, while the pixel as a whole is still perceived in the desired color. Applicants have realized that it is possible to use this freedom regarding the time of color generation for reducing flicker of the display. In practice, many color intensities will be generated by more than one sub-field, causing that the color will be generated at more than one instant. However, the distribution and weights of the sub-fields are such that there will be a subjective luminance peak in displaying such an intensity, e.g., when the highest sub-field is lit, and the instant of this peak will be perceived as the instant at which the color is generated. The latter is in respect to the perception of flicker and not to the perception of color, since, as described above, the time differences are such that the correct color is perceived.
The known device discloses the possibility of having two tables of different combinations of sub-fields for generating the various intensities. However, the combinations of one table are used for one group of pixels and the combinations of the other table are used for another group of pixels. It is to be noted that for an entire single pixel, always combinations from one of the two tables are used for each of its color sub-pixels. Thus, in the known device, a pixel is treated as one object, i.e., its sub-pixels are treated uniformly, regarding the selection of a combination of sub-fields. This contrasts the current invention, where the individual sub-pixels are individually controlled regarding the selection of sub-fields and the subsequent generation of light.
Furthermore, an embodiment of the known device has combinations of sub-fields that are designed in such a way that for many intensity levels, two emission peaks occur during the field period. This is realized by using a relatively large number of sub-fields and by appropriately positioning the multiple high sub-fields in the field period. The occurrence of two peaks in one frame period reduces the occurrence of flicker. In the present invention, a single combination of sub-fields typically has one peak and the other peak is generated by a second color sub-pixel receiving the appropriate sub-field combination with the peak at a different position. The solution of the known device is at the cost of a substantially reduced number of possible intensity levels in relation to the number of sub-fields. This is not attractive since such a large reduction of possible intensity levels seriously reduces the quality of the displayed image. The solution according to the invention results in combinations of sub-fields that can generate more intensity levels than the ones in the known device. This is caused by the fact that in the present invention, the occurrence and position of one peak per combination of sub-fields needs to be controlled, while in the known device, the occurrence and position of two peaks per combination need to be controlled. The latter gives a larger constraint when a combination is created from the available sub-fields, and thus requires a larger number of sub-fields to choose from. The invention provides a larger degree of freedom regarding the creation of combinations of sub-fields resulting in a more efficient use of the number of sub-fields, i.e., in more intensity levels for a given number of sub-fields.
An embodiment of the image display unit according to the invention is characterized in that the selection means is arranged to select, in the situation where the second intensity level is equal to the first intensity level, from the plurality of combinations that are able to realize the second intensity level different respective combinations for the first combination and the second combination. Only controlling the selection of the respective combinations for the first color sub-pixel and the second color sub-pixel in the case where the two sub-pixels have the same intensity, is a relatively easy task. This already leads to a reduction of the flicker in the displayed image.
An embodiment of the image display unit according to the invention further comprises storage means for storing a first set of combinations of sub-fields for realizing respective intensity levels, and for storing a second set of combinations of sub-fields for realizing the same respective intensity levels, wherein the selection means is arranged to select the first combination from the first set and the second combination from the second set. By having two sets of different combinations of sub-fields for generating the possible intensity levels, the selection means merely has to select the combination for the particular intensity level from the set corresponding with the color sub-pixel at hand. This highly reduces the computational effort required at real time, since the sets are created and stored in advance in the display unit.
An embodiment of the image display unit according to the invention is characterized in that a combination of the first set has a first subjective peak in luminance, and a combination of the second set has a second subjective peak in luminance, whereby the first and the second subjective peaks have a time difference of substantially a half frame period. Using the combination of the first set in one sub-pixel of a particular pixel and the combination of the second set in another sub-pixel of that particular pixel, results in the occurrence of two peaks in the frame period at a time difference of half the frame period. This is perceived as a doubling of the frame frequency, resulting in a reduction of the flicker. When this is applied for a frame frequency of 50 Hz, the perceived luminance frequency becomes 100 Hz which is higher than the human eye can see. Thus no flicker will be seen for this particular pixel.
An embodiment of the image display unit according to the invention is characterized in that for realizing a particular intensity level the second combination for realizing this particular level is chosen to be different from the first combination for realizing this particular level. It is relatively easy to generate the two sets wherein the respective combinations for a particular intensity level are different. Using these two sets already results in a reduction of the flicker in displaying the image.
An embodiment of the image display unit according to the invention is characterized in that the combinations of sub-fields for realizing the particular intensity level have been chosen for the first and the second set according to the steps generating a set of candidate combinations of sub-fields, each of which being able to realize the particular intensity level, making a frequency analysis for each pair of the candidate combinations and determine a respective value for respective components having the frame frequency, determining which pair has the smallest value for the component having the frame frequency, and incorporating the candidate combinations of this pair in the first set and the second set, respectively. By analyzing to what extent a pair of combinations to be applied to the first color sub-pixel and the second color sub-pixel comprises a frequency component of the frame frequency, the optimal respective combinations for a particular intensity level are put into the first set and the second set.
An embodiment of the image display unit according to the invention is characterized in that a pixel comprises a green sub-pixel, a red sub-pixel and a blue sub-pixel and wherein the selection means is arranged to select the first combination for the green sub-pixel and the second combination for the red sub-pixel and for the blue sub-pixel. Since a green sub-pixel of a certain luminance contributes to about half the perceived pixel luminance and the red and blue sub-pixels of the same certain luminance together contribute to about the other half of the perceived pixel luminance, flicker is considerably reduced by supplying the first combination to the green sub-pixel and the second combination to the red and the blue sub-pixels.
An embodiment of the image display unit according to the invention is characterized in that the selection means is arranged to select a combination of sub-fields for a neighboring pixel of the particular pixel in dependence on the selection for the particular pixel, whereby for a color sub-pixel of the neighboring pixel corresponding with the first color sub-pixel of the particular pixel, the second combination is selected, and for a color sub-pixel of the neighboring pixel corresponding with the second color sub-pixel of the particular pixel, the first combination is selected. In particular, in the case where an area of the image has a color that is the same as or similar to one of the primary colors, i.e., the color of the sub-pixels, the intensity of one of the sub-pixels will be far larger than the intensity of the others. Then it is not very well possible to compensate the peak generated by one sub-pixel with a peak generated by another sub-pixel according to the invention. By reversing the allocation of combinations in a neighboring pixel, also in this case flicker is reduced since the peak of a certain sub-pixel is now compensated by a peak of the corresponding sub-pixel in the neighboring pixel. This technique can easily be used in the embodiments having respective sets for the first color sub-pixel and the second color sub pixel. Then, in a particular pixel, the first set is used for the green sub-pixel and the second set for the red and blue sub-pixels, while in the neighboring pixel of the particular pixel, the first set is used for the red and blue sub-pixels and the second set for the green sub-pixel. This technique can be implemented by applying a checkerboard pattern to the pixels. Pixels corresponding to white fields are identified as neighbors of pixels corresponding to black fields. Furthermore, the technique can be implemented on a line-by-line or on a column-by-column basis. The pixels of one line (or column) are treated in one way regarding the above selection, while the pixels of the neighboring line (or column) are treated in the second way.
It is a further object of the invention to provide a method as described in the preamble with a reduction of flicker. This object is achieved, according to the invention, in a method that is characterized in that from a plurality of combinations that are able to realize the second intensity level, that combination is selected as second combination in which the subjective peak in luminance is at a different time position in the frame period compared with the subjective peak in luminance in the first combination.